spaco wrote:
From time to time I have had real trouble sharpening SOME drill bits on
my Darex Drill Doctor. I thought I understood it pretty well.
As others have done, I have 2 complete indexes of "Import" bits. I use
them for general work, always replacing them with high quality bits when
the originals fail. (surprisingly, some of the larger sizes do seem to
hold up pretty well).
A couple of days ago I had to drill a couple of #16 holes in some
pretty hard steel, the tang of a knife. First I grabbed a Chinese bit
and it dulled real fast. Then I grabbed a USA HSS dril bit. It drilled
one of the holes and part way through the other before heating up and
dulling.
When I took the Chinese bit to the Darex, I noticed that it was badly
dulled and I couldn't get the angles right. Somehow, it just didn't
look right down in the Drill Doctor cavity where the two little fingers
grab it. I tried several times, but no luck.
The HSS bit sharpened right up with correct angles and I completed
the job for my friend.
After he left, I tried to figure out why I couldn't seem to get the
Chinese bit to sharpen correctly. Finally, I realized that the back
of the "land" had less material in it than the USA bit. These Chinese
bits even feel funny when held in the hand. They feel "skinnier" in
back of the actual cutting land.
This causes the grinding angle to be off a LOT when you transfer the
bit from the locating port to the grinding port. I tried to rotate the
locating collar CCW, but I gave up after several tries and just hand-
sharpened it.
I checked several bits and see the same sort of problem, at least with
the smaller bits, but in no way do I have enough data to tell how
extensive the problem might be.
I put an image of a drill bit on my website to try to show this problem.
Go to:
http://www.spaco.org/BadDrillBits.jpg
The RED areas show where, IMO, metal is missing from the back side of
the land area.
I'd be interested to hear from others who have had similar experiences
or could shed light on the problem.
I have seen cheap drill bits with proportions
that just didn't look right. I suspect it's
the same issue as you found.